The present invention generally relates to the art of controlling specific characteristics of materials, and more particularly pertains to oxidizing and/or altering the tack of contaminants found in an aqueous stream, such as water, whitewater, and pulp slurries.
People throughout our society have become increasingly aware and concerned about the environmental issues that plague the world. The depletion of the ozone layer, the rain forests, and clean water are just a few of the environmental issues that are being addressed. One approach in addressing these issues includes preserving resources by recycling them. Consequently, the recycling industry has become instrumental to serving this need.
In the recycling field, one area of increasing interest is the reuse of wastepaper. Millions of tons of wastepaper are generated every year in the United States. Recycling this wastepaper can save countless trees, as well as provide other ecological and economic benefits. However, the key to reuse of this wastepaper is the removal of contaminants from the wastepaper, thereby facilitating the use of recycled or secondary fibers from the wastepaper.
The paper recycling industry encounters a variety of contaminants in wastepaper. Many of these contaminants adhere to paper fibers and therefore may cause problems during the recycling process. One such contaminant is xe2x80x9cstickiesxe2x80x9d, which were used originally as paper adhesives or tacky adhesives. Stickies typically are classified as hot melts, pressure-sensitive adhesives (PSAs), latexes, and binders. Pitch is another contaminant associated with both virgin and secondary fibers. Pitch is a part of the extractives from wood, and is released during pulping. Other contaminants include ink, toners, sulfides, iron, and any other substance that may be found in water, and particularly a fibrous stream.
Contaminants may cause operational arid product quality problems. Specifically, contaminants may be deposited on wires, felts, press rolls, and drying cylinders of paper machines. In addition, contaminants may hinder bonding of fibers, increase web breaks, and reduce product quality in the papermaking process. Consequently, contaminants must be controlled in order to improve papermaking operations and product quality.
Tack is the sticky property of paper adhesives, paperboard adhesives and glue coating materials. The tack of an adhesive and the adhesive""s ability to bond to another surface is dependent, in part, upon the surface energy of the adhesive. Reducing the tack of contaminants can minimize the propensity of the contaminants to attach to paper machine surfaces, thereby leading to fewer operational problems.
Various prior art methods are used to reduce the tack of the contaminants. Some methods use repulpable or recyclable adhesives. More common methods include chemical additives for modification, detackification, or pacification of the contaminants. For instance, detackification of contaminants is frequently accomplished by adding minerals, such as talc, or surface-active chemicals. These minerals and surface-active chemicals attach to the surface of the contaminants and alter their surface properties, thereby causing tack reduction. This method of tack reduction is described in a publication entitled, xe2x80x9cSuccessful Approach in Avoiding Stickies,xe2x80x9d by S. Abraham, Tappi J., 81:2 79-84 (1998), which is incorporated herein by reference. Nonetheless, chemical additives can be very expensive and may cause other problems in the papermaking process, such as a decline in product quality.
Mechanical methods for controlling contaminants include dispersion, screening and cleaning. Dispersion is a technique by which contaminants are broken up into smaller and smaller particles until they are invisible in the final product. Unfortunately, the overall appearance of a product may be diminished greatly by the presence of contaminants. In addition, when the product containing contaminants is wound, sticking may occur between adjacent layers.
Screens and centrifugal cleaners are typically used to remove stickies, pitch and debris from the fiber stream. In general, screens are used to physically separate fiber from contaminants based on the differences between the sizes and shapes of contaminants and the holes or slots in the screen. One problem is that screens cannot remove contaminants that are either smaller than the size of the screen hole or deformable enough to pass through the screen hole. Centrifugal cleaners separate contaminants from fiber primarily on differences between the specific gravities of the fiber and the contaminant. However, separation is poor if the specific gravity of the contaminant is similar to the specific gravity of the fiber.
In an article by Lang et al., a photolytic process is used to oxidize trinitrotoluene. P. S Lang, W-K Ching, D. M. Willberg, M. R. Hoffman, xe2x80x9cOxidative Degradation of 2,4,6-trinitrotoluene by Ozone in an Electrohydraulic Discharge Reactor,xe2x80x9d Environ, Sci. Technol., 32, 3142-3148 (1998). Specifically, trinitrotoluene is dissolved in water in a 4-liter reactor and oxidized as a result of light emitted from an electrical discharge. Similarly, in an article by Willberg et al., a photolytic process is used to oxidize 4-chlorophenol (4-CP). Specifically, 4-CP is dissolved in water in a 4-liter reactor and oxidized as a result of light emitted from an electrical discharge. D. M. Willberg, P. S Lang, R. H. Hochemer, A. Kratel, M. R. Hoffman, xe2x80x9cDegradation of 4-chlorophenol, 3,4-dichloroaniline and 2,4,6-dinitrotoluene in an Electrohydraulic Discharge Reactor,xe2x80x9d Environ, Sci. Technol., 30, 2526-534 (1996). The distance of the sides of the vessel from the source is estimated to be 8 cm. Lang et al. and Willberg et al. both discuss that the oxidation process occurs in the plasma channel between and in the immediate vicinity of the electrodes, and as a result of a photolytic process induced by light emitted from a discharge between the electrodes.
Therefore, there is a need for a system and method for improving the removal efficiency of contaminants, such as stickies and pitch, from a fiber stream. In addition, there is a need for a system and method that can oxidize and/or detackify contaminants by altering, without the use of chemicals, the surface properties of the contaminants. Also, there is a need for a system and method for oxidizing materials at great distances for applicability in large-scale industrial settings. There is yet another need for a system and method that can alter the surface properties of materials inexpensively and simply.
The present invention solves the above-described needs by providing a system and method for oxidizing a material by exposing the material to an electrical discharge in a liquid medium.
One method for oxidizing a material in a fiber stream comprises the steps of providing a fiber stream; and causing oxidation of a material in the fiber stream by producing a predetermined amount of electrical discharge in the fiber stream. The material in the fiber stream is oxidized such that the zeta potential of the material in the fiber stream is reduced and/or the tack of the material is reduced.
The material can be ink or toner, where the ink or toner is oxidized such that the ink""s or toner""s ability to adhere to a fiber in the fiber stream is reduced. The material can be a sulfide, where the sulfide is oxidized such that the odor of the fiber stream is reduced. The material can be a polymer used as an adhesive, such as stickies, or it can be pitch, where the polymer or pitch is oxidized such that the tack is reduced.
Another method for oxidizing a material in a liquid medium comprises the steps of providing a liquid medium; providing a material in the liquid medium; and introducing a predetermined amount of electrical discharge in the liquid medium to cause oxidation of the material, said oxidation occurring independent of a light source. The electrical discharge in the liquid medium can occur periodically.
The liquid medium is selected from a group consisting whitewater, water, and a pulp slurry. The material is oxidized such that the zeta potential and/or tack of the material is reduced or the odor of the liquid medium is reduced depending on the material that is oxidized.
An alternative method for oxidizing a material in a liquid medium, comprises the steps of providing a liquid medium containing a material; and introducing a predetermined amount of electrical discharge from a discharge source in the liquid medium to cause oxidation of the material at a predetermined distance in the liquid medium from the discharge source, where light emitted from the discharge source is negligible. The predetermined distance can occur beyond 8 cm from the discharge source and typically, may be at a distance ranging from about 10 cm to about 4.5 m from the discharge source.
These and other objects, features, and advantages of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the appended drawings and claims.